The present invention relates to injection devices for injecting medicament in fluid form having high viscosity which means that these devices require high forces in order to press the fluid through a needle when injecting the medicament.
Auto-injectors or pen-injectors have been on the market for many years. One of the first auto-injectors was developed for war-times, which was activated by pressing the injector against a body part for activating it. The main concern was to have the medicament injected as fast as possible, without much concern for the patient or for handling safety aspects. During recent years some medicaments have been developed such that these have to be injected by the patients themselves. Therefore, depending on the intended use and type of medicament, injection devices having a varying degree of automatic functions that facilitate the injection of medicaments in a reliable and safe way for patients and even for trained personnel; e.g. physicians, nurses, have also been developed.
Auto-injector devices having an automated injection function often comprise a housing, a spirally wound compression spring acting on a plunger rod which in its turn acts on a stopper inside a medicament container for expelling the medicament through a needle attached to the container. Normally, one end of the spring often abuts an inner end surface of the housing, which means that the housing has to be dimensioned to the force of the spring. When fluids with high viscosity are to be injected using an auto-injector, high forces are required to expel the medicament through a fine needle. Consequently, the spring becomes very large both regarding the diameter of the wound spring and also the diameter of the thread of the wire. The size of the spring means that the device becomes large, and for some applications and customers, such device sizes are not acceptable.
Furthermore, in particular when injection of medicament takes a long tome, for example when a high-viscosity liquid is injected and/or a needle having a small diameter is used, the user of the injection device wants to know whether or not the injection is still proceeding. In other words, the user self-administering a medicament needs to be informed when the injection is completed and it is safe to remove the injector from the injection site.
WO 2004/020028 describes an injection device having a closed scale band. When administering a product, a required dosage is set on the injection device by rotating a rotational button. This transports the scale band in accordance with setting a dosage, such that once the dosage setting rotation is complete, the corresponding dosage unit can be read through a window. Such scale band for dosage indication does however not show the progress of injection.
An injection device having a band type indicator as suggested in WO 2004/020028 is, however, complex to assemble. Moreover, the movement of the band is visible to the user from one direction or viewing angle (or small band of viewing angles) only. Furthermore, slow injections may be difficult to detect by a user.
Another means of information regarding the progress of an injection is by tactile means. Document WO 2008/083875 discloses a medical delivery device comprising a tactile information means capable of providing vibration to the device during e.g. an injection cycle. The vibration is preferably generated by an unbalance member. The tactile information member of the device is rather bulky, especially if it is to be fitted into a rather small device, at the same time as the space usually is limited. Also, vibrations of the whole device may be undesirable in some instances and for some patients, desiring more subtle tactile information.